Power hop for agricultural tractors is a well known phenomena, as described by B. P. Volfson and M. Estrin in “The Slip-Stick Phenomenon in Vehicle Ride Simulation”, published by ASME “Computers in Engineering” 1983 Vol. 1. Power hop occurs most commonly with drawn implements in dry soils. Power hop can also occur on hard surfaces such as concrete or asphalt. It is believed that the power hop condition occurs when the traction force exceeds the stable limit of the tire-to-ground interface. When the traction load/pull reaches this limit, a stick-slip phenomena occurs at the tire-to-ground interface which excites the vehicle bounce and pitch natural frequencies. The resulting bounce and pitch motion is called power hop. Power hop can be pure pitch, pure bounce, or a mixture of bounce and pitch. In some cases, the bounce and pitch motions can become severe enough to cause operator discomfort, decreased implement performance, and equipment damage.
Often when power hop develops on an agricultural tractor, the traction force must be brought to zero in order to stop power hop bounce and pitch oscillations. Simply reducing the traction force on the tires may not be enough to return to stable traction after the tractor bounce and pitch resonant frequencies have been excited. An operator is often required to raise the implement or stop the tractor to stop power hop oscillations. An automatic means to reduce traction force and stop power hop bounce and pitch tractor oscillations without lowering productivity is needed.
An experienced operator may manually reduce the traction force by slowing down the tractor or raising the implement, since the traction force requirement is proportional to velocity and cut depth for many tillage applications. However, the operator has no way of knowing where to set the power level for stable traction. Usually a trial and error approach is used, causing the operator to experience power hop as field conditions vary. If the power level is set too high, the power hop condition will be present. An automatic means to lower the tractor steady state power level to maintain stable traction is needed.
Attempts have been made to control power hop by adjusting the drawbar height, tire inflation pressure, and weight/ballast distribution. For example, European patent application EP 1 022 160, published on 26 Jul. 2000 and assigned to the assignee of this Application, describes a wheel mounting disk which is intended to more accurately center a wheel and tire on a vehicle axle. However, such a wheel mounting disk has not completely eliminated road lope. These adjustments help by changing the traction limit slightly and/or by reducing the vehicle bounce and pitch motion amplitudes. However, the physical traction limit still exists, and when exceeded, the vehicle will power hop.
It has been proposed to avoid power hop by limiting the traction force of the vehicle, such as with the traction control and anti-lock bake systems available on many automobiles today. In these systems, the traction force is reduced when wheel slide is detected by a wheel speed sensor. U.S. Pat. No. 6,401,853 describes a system which detects power hop using wheel speed sensors on accelerating over-the-road trucks and limits the traction force by controlling engine torque.
However, the torsional compliance of a pneumatic agricultural tractor tire is large when compared to that of an automobile tire. For this reason, the automotive and over-the-road truck approach of measuring wheel stick-slip characteristics with a wheel speed sensor is not practical for an agricultural tractor. When stick-slip is occurring at the tire-to-ground interface, the tractor axle speed may be constant. Some other suitable means for detecting power hop on an agricultural tractor is needed.
U.S. Pat. No. 6,035,827, issued 14 Mar. 2000 to Heinitz et al., discloses a system wherein an engine speed signal is fed back to a characteristic function, and wherein an engine fuel quantity signal is operated on by an inverse transmission function to generate a compensated fuel quantity signal. However, the practicality of such a system is doubtful because it is believed to be difficult to extract the needed information from the feed back engine speed signal.
German Published, Non-Prosecuted Patent Application DE 195 37 787 A1 discloses a method for compensating bouncing oscillations. In that reference a signal expressing the wish of the driver is filtered through the use of a transmission element. The parameters of the transmission element, and thus its transmission behavior, are changed as a function of operating parameters while the internal combustion is operating. Furthermore, a subordinate rotational speed control is used for non-steady operation, which leads to a considerable number of application parameters.
U.S. Pat. No. 6,589,135, issued to Miller and assigned to the assignee of the present application, describes a proposed system an method for reducing vehicle bouncing wherein the engine power or driveline torque is varied in order to counteract vehicle pitching. However, this system did not sense both vehicle bouncing and vehicle pitching, and it was found that this system would not completely eliminate vehicle bouncing.
U.S. Pat. No. 5,819,866 is believed to describe a system for controlling pitching in a vehicle operating at transport speeds wherein pitching is detected by accelerometers.
It would be desirable to have a system which controls both vehicle bouncing and vehicle pitching in an agricultural tractor operating at low speed and heavy load conditions.